Angry Christians protested across Pakistan on Monday to demand better protection after a devastating double suicide bombing at a church killed more than 80 people.

The attack on All Saints church in the northwestern city of Peshawar after a service on Sunday is believed to be the deadliest ever to target the country's small Christian minority.

Christians demonstrated in towns and cities across Pakistan, including Islamabad, Lahore, Karachi and Peshawar, demanding better protection from authorities.

A Muslim man was killed in Karachi when scuffles broke out at a Christian protest outside a mosque, police said.

In the city's eastern neighbourhood of Korangi, police used tear gas and fired aerial gunshots to disperse protesters, who set fire to a house and other properties.

"Some protesters pelted stones at some shops that ignited a brawl and scuffle between the two groups (of Christians and Muslims) who also attacked each other with stones," police chief Muneer Shaikh told AFP.

"We have arrested 45 suspects who were agitating in the area and the situation is now under control."

More than 600 protesters blocked a major highway in Islamabad for several hours during the Monday morning rush hour, burning tyres and causing long tailbacks, an AFP photographer said, and later about 2,000 people gathered to protest outside parliament.

In Peshawar, about 200 demonstrators took to the streets, smashing windows at the main Lady Reading hospital, where many of the victims were treated, and blocking the main Grand Trunk road.

In front of All Saints church, more than 100 people chanted slogans demanding justice and attacking the national government for failing to protect Christians.

And they had harsh words for cricketer-turned-politician Imran Khan, whose Pakistan Tehreek-e-Insaf (PTI) party runs the provincial government in northwestern Khyber Pakhtunkhwa province.

"Imran Khan and his senior deputy have failed to protect Christians at their praying centres," Khalid Shahzad, who lost five family members in the attack, told AFP.

"The government of Khyber Pakhtunkhwa and Imran Khan are just making slogans, there is nothing practical (to protect us). They do not have any sympathy for minorities."

The death toll from the blasts rose to 82 on Monday, according to medics, with about 130 in total wounded.

Senior Peshawar police official Najeeb-ur-Rehman said security around churches in the city would be stepped up, but survivors of the bombing spoke of their fears of further violence.

"We had very good relations with the Muslims -- there was no tension before that blast, but we fear that this is the beginning of a wave of violence against the Christians," Danish Yunas, a Christian driver wounded in the blast, told AFP.

The United States described the attack as "heinous", saying it remained concerned about continuing sectarian violence in Pakistan.

"Extremist violence against innocent Pakistani men, women, and children of any faith is an assault on universal human values," said US State Department spokeswoman Jen Psaki.

The small and largely impoverished Christian community suffers discrimination in the overwhelmingly Muslim-majority nation but bombings against them are extremely rare.

The 400 or so worshippers were exchanging greetings after the service when the bombers struck, littering th

e church with blood, body parts and pages from the Bible.

The walls were pockmarked with ball bearings that had been packed into the bombs to cause maximum carnage in the busy church.

Sectarian violence between majority Sunni and minority Shiite Muslims is on the rise in Pakistan but Sunday's bombings will fuel fears the already beleaguered Christian community could be increasingly targeted.

A faction linked to the Pakistani Taliban on Sunday claimed the attack, saying it was to avenge US drone strikes on Taliban and Al-Qaeda operatives in the country's tribal areas along the Afghan border.

But on Monday the main spokesman for the umbrella Tehreek-e-Taliban Pakistan (TTP) group said they were not responsible, adding that it was "an attempt to sabotage the atmosphere" of proposed peace talks.

Prime Minister Nawaz Sharif has called several times for talks with the Taliban and two weeks ago won backing from the country's main political parties.

But speaking in London on Sunday he said the government was "unable to proceed further" with talks following the church attack.

Pakistani Christians, who make up just two percent of the country's 180-million population, often lead a precarious existence, many living in slum-like "colonies" cheek-by-jowl with Muslims and fearful of allegations of blasphemy, a sensitive subject that can provoke outbursts of public violence.